Wide-band amplifier



Noi'. 13, 1951 L. w. HERSHINGER WIDE-BAND AMPLIFIER Filed May 26, 1950 INVENTOR.

HGf/WI! m am w. HMM/06m I I I I I I I I I I Ik Q E Ioww) Mv/Z- Patented Nov. 13, 1951 UNITEDST1-Aflfas,r vPllTILNT* OFFICE "PhilcoCorporation, `lhladelphia, Pa., a cor;

lioiation l of Pennsylvania v, VAppliauch Mayas, 1950, Vserial No. 164,452

1 The present invention relates to a high-gain multi-stage video amplifier capable of passing frequencies down to and including direct current. It is particularly suited for use in television receivers Where it is desired to retain the D.C. component of the video signal ywhich is normally lost in conventional amplifiers employing resistance-capacitance coupling.

It is advantageous to incorporate in television Vreceiving apparatus a vseries of-amplifler stages 'A still further advantage achieved by the `use of a direct-coupled video amplifier is that the autoL matic gain control circuit may be connected to the output of the video amplifier instead of-to the output of the video detector, thus giving 'a iiatter AGC characteristic than is customarily obtainable.

While the benefits of direct-*coupled video amplifiers have long been recognized, such circuits have not achieved wide popularity due to the fact that a number of drawbacks have heretofore been inherent in their use. Perhaps the principal disadvantage has been Ithat variations in the power supply of the receiver have caused corresponding variations in amplifier output. This is especially true, for example, in Aarrangements Where the output voltage of one tube is applied directly to the control grid of the following tube. It has been found insuch cases that not only are the component values quite critical, but also that small fluctuations in. plate supply voltage result in an unbalance of the amplier. Furthermore, this type of circuit places a relatively high D.C. potential on the amplifier tube control grid, which may lead to distortions in the signal wave-form. Some of these factors might be overcome by the use of a separate power supply for each tube, or by the use of complex regulating equipment. However, these expedients would increase the cost of the television receiver to a prohibitive degree.

One object of the present invention, therefore, is to provide a high-gain direct-coupled video ampliiier particularly adapted for television purposes.

Another object of the ,present invention is to provide a wide-band multi-stage ampler cir- 1u claim.' (c1. 1794-171) cuit wherein the A.C. and D.C. components of the input signal are transmitted through the amplifier by different paths.

A further object of the present invention is to provide a direct-coupled video amplifier suitable for use with input signals which include a D.C. component, and to provide means for establishing the A.C. gain of the circuit substantially independently of the D,C. gain whereby a substantially balanced condition may be reached.

Other objects and advantages will be apparent from the following description and from the drawing, the single figure of which illustrates a television receiver incorporating a preferred form of video amplifier designed in accordance with the present invention.

Referring now to the drawing, there is shown a television receiver of the superheterodyne type. This receiver includes an antenna I 0 which is connected to the input terminal of a converter I2. The latter also receives the output of a local oscillator I4, which beats with the incoming energy received by antenna l0 to develop an intermediate frequency (IF) signal. This signal is then fed to an IF amplifier I6.

The output of the 1F amplifier I6 contains both the audio and video information present in the composite television signal. The audio information is segregated by any suitable type of separating circuit (not shown) and applied in the usual manner to an audio reproducing apparatus. The remaining portion of the signal passes through a video IF amplifier I B and is delivered to a video detector 20. The latter acts to demodulate the picture information, which is then applied by Way of a multi-stage video amplifier 22 tothe grid 24 of an image-reproducing cathode-ray tube 2li.l A synchronizing separator circuit 28 is also connected to the video amplifier 22,*so that the horizontal and vertical synchronizing pulses contained in the composite video signal may be separated therefrom and utilized to control line and eld deflection of the electron scanning beam of the cathode-ray tube 26 through the action of the usual scanning generators (not shown). Furthermore, an AGC potential is derived from the output of the video amplilier 22 andA applied by Way of an AGC detector 29 and a conventional filter circuit 30 to the IF amplifier units I6 and I8 so as to control their gain in some suitable manner.

In describing the present invention, it will be assumed that the composite television signal, including both video information and synchronizing pulses as well as a D.C. component, has

passed through the IF amplifier I6 and the video amplifier I8 and is received by the video detector 20. This circuit acts in a more or less standard fashion to demodulate the video signal through the action of a diode detector tube 32, with the synchronizing pulses extending in a direction of negative polarity. This detected signal is then appliedilto vthe contr,ol-.grid*3'4 of a itriode 1136 forming part of 'the video ampler unit i22.

The triode 36 has its anode 38 connected to the terminal 46 of a positive potential source through two resistors 42 and 44. The .resistor 44 acts as a D.C. load for the trio'de "36, and hence is by-passed to ground :forthe A.-C..components of the output signalL-by means.of.a.ca pacitor 46. resistors 42 and 44 is at A.C.ground po,tential, the only voltage variation appearing at this point represents changes in the D.C. component `of the output signal from triode 36. The resistor 42,however, acts as Pa loadforfthe A..C.fcom :ponen't Voi :the signal, and hence the -A.-C.signal voltage may be derivedfromthe anode 330i the triode.

'There-C. and D.-'C.componentsfof the signal .output of `triode'fi, which,\as above stated, appear. respectively .at the anodef38 and lat the '46 point between the 4resistors 4-2 and .44, Vare `conducted by separate paths 'to apentode @56. The A.C. 'components of :si-gn'al voltage -pass through a capacitor. 52 to thefcontrol grid'54 of fthe pentode, while 1then-C. voltage'variations are applied directly lto.its\s creen grid 5.6. Con- .'trolgrid 54 is returned throughagridresistor f58 to a `source-cf *negative .potential connected to the terminal 5T. Anode-ifof-thepentode is connected ',to terminalrll of the ,operating potential source through-two resistors 66 and.-62, ythe .junction :point 364 of 'these fresistors being 'grounded v.through thecapacitorGE. The;D.- C. plateiload'of pentodezi is constitutedof both the lresistors. l iand'62 in'seriesfwhile the A.C. plate `load'cornprises only fthe single resistor i60. An- .ode 53501 pentode 5! is further connectedfto the control grid 24 of :the :cathode-ray tube 26 through a parallel resistance-capacitance net- 4work 968. Operating 'bias =for the `cathode-ray tube ygrid 24 is supplied from-point n'Et'on azvoltage-divider network consisting of the resistors f6'2,"69,'the resistor of the RC networkffand; a vfurtherresistor 'i2 all connectedin series between positivete'rminalflland the `.terminal r'14 'cfa source. of negative potential.

:It .will thus be seen thatthe output voltageiof the triodef36 is applied to thepentode over amplier ,22 is .substantially equal toits-D.C.

gain,.and can be obtained, for example, by making the value 0f the D.C. load resistor 62-suficiently high so that the D.C. amplication of the pentode Resistor 42:3300 ohms .Condenser 44:8.microfarads Condenser' 52:.2 micro'farad Resistor 60:3900 ohms Condenser 66:8 microfarads "Resistor 62:12.000 ohms l-15 Since the junction point 48 of the Potential at terminal =+300 volts .Potential'atterminal 51=6 volts Potential at terminal 14:-300 volts It will of course be understood that the principles of applicants invention y as exemplied 1 by the -video amplifier '22 -may i be utilized ,in other ynet-works #and vvfor other .applications than `ythe .oneA illustrated and s described. .The conceptican advantageously ;,be employed -in .a wide Ivariety of circuit arrangements in whichA an intelligence vsignal'hailing-4al D.C. factor is lto bepassedwith equally high gain for -eachI of .its components.

Having thus described ram-invention@ claim;

An; amplierfcomprising lfirst andsecond electronA discharge tubes, the rstof-said tubes fhavingzat least-a cathode, anfanodefandn-a grid-the -secon'dy of saidtubesfhaving at least-acathodeyan anode,- andrlrst vandsecontgrid; electro des; means -forrapplyng -to-.tlreggrid cathodecircuit of said rzst tube an-electricalf signal -having both A. C. ,and,D.-.C.z.components, a nairfcfload resistors; and a -souricetof anode; currentgseriallyrcennected in the anode-cathode:circuitiof 'Saidzirst tuberaiby-pass capacit/,or connected between apcint of xed potential fand.the-junction-between said load resistors, means including a capacitor connected ibetween :the-anode o f ysaid nest tube and-.the rst grid :of ffsaid :Second tube for ferilli/iris fv the {Ah-.C-

LrNooLNWxHEReHINGER.

REFERENCES CIT-ED Thefiollowlngeferences are of record vin 'the lile of this patent:

UNITED -sfTA'rEs PATENTS Number -Name f Date i 2,211,914 Soller Aug. 20, 11940 123075387 i'Blumlein v Jan. 5, `14943 .V25-134,939 '1-Levy -rJ an. 527,1 11948 12,5413198 -Brenholdt ?Feb."l3,1 19'51 

